fix(netbsd): enable the rule only for NetBSD.

src/output/mod.rs

@@ -1,4 +1,4 @@
-#[allow(unused_imports)]
+#[cfg_attr(target_os = "netbsd", allow(unused_imports))]
 pub use self::cell::{DisplayWidth, TextCell, TextCellContents};
 pub use self::escape::escape;